The exhaust emissions from motor vehicles are a well known and significant source of air pollution. The most significant gaseous vehicular emissions comprise pollutants such as carbon monoxide (CO), oxides of nitrogen (NO and NO2 collectively NOx), and unburnt hydrocarbons (HC). In addition to the gaseous components, the diesel exhaust stream also contains entrained solids, commonly referred to as particulate matter or soot, upon which may additionally be adsorbed volatile/soluble organic fraction (SOF). The gaseous pollutants have been demonstrated to be major contributors to the photochemical smog and ozone events which have been correlated to significant adverse impacts on human health (M. V. Twigg, Applied Catalysis B, vol. 70, (2007), p 2-25). Additionally soot and its associated SOF content present a further health risk as its small size makes this potentially carcinogenic material respirable. Hence increasingly stringent legislative limits have been introduced in order to regulate the emissions from both gasoline and diesel internal combustion engines e.g. Euro 5 or Euro 6, Regulation (EC) No 715/2007 of the European Parliament and of the Council, 20 Jun. 2007, Official Journal of the European Union L 171/1, see also Twigg, Applied Catalysis B, vol. 70 p 2-25 and R. M. Heck, R. J. Farrauto Applied Catalysis A vol. 221, (2001), p 443-457 and references therein.
An advantage of diesel/compression ignition engines is their inherent lean burn operation, i.e. combustion occurs due to fuel compression under conditions of excess O2, which results in increased fuel economy and decreased emissions (in g/km) of CO, HC and CO2 cf. the stoichiometric gasoline engine. Moreover diesel engines offer increased durability and are able to provide high torque at low engine rpm and these attractive characteristics have helped diesel passenger cars gain a >50% market share in the Western Europe market. However despite the inherent advantages offered by lean mode combustion conventional diesel engines still do not meet the aforementioned legislative targets and thus a range of exhaust after-treatment technologies have been developed to address this requirement. These technologies include, but are not limited to, engine control methodologies/modification, alternate combustion cycles and the use of after-treatment systems e.g. catalytic control devices which eliminate exhaust pollutants by promoting chemical changes to convert unwanted compounds into more benign species. In the case of diesel/compression ignition engines the latter devices include the Diesel Oxidation Catalyst (DOC), Diesel NOx Trap/NOx Storage Catalyst (DNT/NSC) and Selective Catalytic Reduction catalyst (SCR) to address emissions of CO, HC (DOC) and NOx and the use of the Catalysed Diesel Particulate Filter (CDPF) for the removal and combustion of entrained solids, commonly referred to as particulate matter or soot.
Of the various catalytic emission reduction systems for diesel emission control listed above, the DOC is both the most widely studied and implemented technology, for examples see U.S. Pat. No. 5,371,056, U.S. Pat. No. 5,462,907, U.S. Pat. No. 6,153,160, U.S. Pat. No. 6,274,107, J. A. A. van den Tillaart, J. Leyrer, S. Eckhoff and E. S. Lox in Applied Catalysis B vol. 10, 1-3, p 53-68 (1996). The design of these systems and materials employed therein are somewhat generic and typically consist of a refractory oxide support e.g. an (modified) alumina, a hydrocarbon storage/release component and an active Precious Group Metal (PGM) to catalyse the oxidative conversion of the pollutants into more benign products (H2O and CO2).
The use of the HC storage/release component, conventionally a microporous crystalline aluminosilicate also known as a Zeolite or molecular sieve, is employed to prevent low temperature HC poisoning of the PGM centres (see Applied Catalysis B, vol. 70, (2007), p 2, Applied Catalysis A vol. 221, (2001), p 443). The introduction of the Zeolite in the DOC provides a mechanism for the low temperature condensative adsorption of a significant portion of the higher molecular weight unburnt HC species emitted during ‘cold start’ of the engine. This limits the potential for HC adsorption on active precious metal centres and their resultant poisoning by ‘site-blocking’. As the exhaust gas temperature increases the retained HC species are ‘released’ by evaporation and diffusion out of the porous structure of the Zeolite but only at temperatures where the PGM is fully active and capable of combusting the plume of released species (see U.S. Pat. No. 2,125,231).
The choice of metal(s) in the DOC is based upon their ability to offer the highest turnover frequency (number of reactions per second) with respect to the oxidation of CO and Hydrocarbon to CO2 and H2O at low temperatures and low concentrations of active component within the DOC formulation. Initially Pt (e.g. U.S. Pat. No. 5,627,124) or more recently the combination of Pt and Pd has been employed as the primary catalytic species (e.g. US2008/0045405 A1, U. Neuhausen, K. V. Klementiev, F.-W. Schütze, G. Miehe, H. Fuess and E. S. Lox in Applied Catalysis B: Environmental, vol. 60, 3-4, (2005), p 191-199 and references therein).
However, the operational requirements of the DOC with respect to conversion of gaseous emissions have been augmented over time to meet specific new challenges arising from successive generations of legalisation, e.g. the ability to efficiently combust post-injected HCs to generate the thermal ‘bloom’ required to initiate DPF regeneration or more recently the ability to oxidize NO to NO2 in order to facilitate low temperature NH3-SCR chemistry. Moreover, this extensive multi-functionality must be realised without detrimental impact to the primary role of the DOC for effective emission control i.e. the DOC must posses a low temperature ‘light off’. Thus in addition to such multi-functionality the DOC must provide operation at low temperatures to minimise ‘cold-start’ emissions. This requirement is especially critical given the increasingly lower temperature window of operation of the current and next generation diesel engines, which present increasing CO and HC emissions arising from the increased use of exhaust gas recirculation or advanced combustion cycles employed to decrease engine out NOx levels (e.g. WO/2005/031132, Method and Apparatus for Providing for High EGR Gaseous-Fuelled Direct Injection Internal Combustion Engine).
These multiple challenges are rendered yet more difficult due to the intrinsic process of CO oxidation. The catalytic conversion of this pollutant is known to follow a negative order kinetic response i.e. higher concentrations of CO are self-inhibitory to the rate of oxidation (see A. Bourane and D. Bianchi J. Catalysis 222 (2004) 499-510 and references therein). Moreover during reaction there is competition for adsorption centres on the active PGM between CO, NO, HC and O2, with the adsorption of HC being particularly problematic as this acts as a poison to low temperature operation, necessitating the inclusion of the aforementioned zeolite component to address this issue. Hence as engine out concentrations of CO and HC increase and engine out temperatures decrease the challenge for effective operation of the catalyst to fulfil legislative requirements becomes increasingly difficult. A further and final requirement is that the DOC must maintain high levels of activity through end-of-life. This is a stringent requirement given the DOC is exposed to transients of extreme temperatures in the presence of steam e.g. for the operation of a close-coupled catalyst (to minimize light-off time) or during the active regeneration strategy required for the DPF. The latter case is especially challenging since there is an in-catalyst exotherm and steam generated by the combustion of post-injected hydrocarbons specifically located at the dispersed PGM sites. Such processes are known to result in catastrophic sintering (PGM particle coalescence), particularly for Pt (see M. Chen, L. D. Schmidt, J. Catal. 56 (1979) 198). This has resulted in the aforementioned introduction of Pt/Pd bi-metallic DOCs with a primary role of Pd to inhibit the rate of Pt sintering (Lox et al. Applied Catalysis B: Environmental, vol. 60, 3-4, (2005), p 191).
However, despite these many developments, it has still been necessary to increase the PGM content of conventional DOCs in order to fulfil the emission targets for end-of-life performance. This in turn has increased demand for Platinum (Pt) and Palladium (Pd) resulting in price pressure for these PGMs and a resultant impact on the cost of emissions aftertreatment systems for the vehicle manufacturers and end customer. The relative costs of Pt versus Pd (average price Jan. 1, 2008 through Jan. 1, 2009 was 1535 US dollars and 342 US dollars respectively) means there is a continuing incentive to enable the use of higher Pd, lower Pt content DOC technologies as a means of decreasing total system cost. However, to date, practical limitations have been encountered regarding the usage of lower Pt:Pd ratios. While Pd is effective at enhancing the stability of PGM dispersion it has been found that there appears to be a practical limit beyond which CO and HC oxidation performance are below the levels required to fulfil emissions targets (US2008/0045405 A1). Additionally it is known that Pd enhances the sulfur sensitivity i.e. poisoning of the DOC, resulting in retardation of catalyst light-off (see, The Impact of Sulfur in Diesel Fuel on Catalyst Emission Control Technology—MECA white paper Mar. 15, 1999 and references therein at www.meca.org). Finally Pd has also been found to inhibit the oxidation of both paraffins and NO, the former issue resulting in increased tailpipe HC, while the inhibition of NO2 generation is linked to decreased performance for both CDPF and urea-SCR systems (US2008/0045405 A1).
Hence what is required in the art is a method or technology to enable the use of high Pd content, more cost effective, diesel oxidation catalysts to replace conventional high Pt content technologies. These high Pd DOCs must offer competitive fresh, de-greened, hydrothermally durable and poison resistant activity under the diverse conditions of the diesel exhaust environment while enabling superior performance at equal metal cost or equal performance at decreased cost. Additionally they must provide the aforementioned improvements whilst retaining high NO and HC oxidation functionality as required in modern multi-brick emission control architectures.